Preheat oven to 375 degrees. Spray a baking pan dish with cooking spray.
Using the slicing blade of your food processor or a sharp knife, slice potatoes thin.
Heat olive oil in a medium saucepan and add onion. Cook for 5 minutes. Add garlic and cook one more minute. Add milk, cream, salt and pepper and cook for 10 more minutes over low heat. Add goat cheese and cook until creamy.
Arrange half of the potatoes in your baking dish and top with 1/3 of the cheese mixture. Arrange remaining potatoes on top and add remaining cheese mixture. Sprinkle with Gruyere Cheese.
Cover with foil and bake for 50 minutes. Uncover and bake 30 minutes longer, or until potatoes are tender. Garnish with thyme and serve.